title: The catalytic mechanism of kynureninase from Pseudomonas fluorescens: insights from the effects of pH and isotopic substitution on steady-state and pre-steady-state kinetics.
abstract: The effects of pH and isotopic substitution of substrate and solvent on the reaction of kynureninase from Pseudomonas fluorescens have been determined. The pH dependence of kcat/Km for L-kynurenine is bell-shaped, with apparent pKa's of 6.25 +/- 0.05 on the acidic limb and 8.9 +/- 0.1 on the basic limb, and with a pH-dependent value of kcat/Km of 2 x 10(5) M-1 s-1. The pH dependence of kcat/Km for 3-hydroxykynurenine is also bell-shaped, with apparent pKa's of 6.49 +/- 0.07 and 8.55 +/- 0.09, and with a pH-dependent value of 2.5 x 10(3) M-1 s-1. The kcat for L-kynurenine decreases at acidic pH values, with an apparent pKa of 6.43 +/- 0.06 and a pH-dependent value of 7 s-1. The solvent kinetic isotope effect on kcat for the reaction of kynurenine in [2H]H2O is 6.56 +/- 0.59, whereas there is no normal kinetic isotope effect on kcat/Km, at pH 8.1. The proton inventory of kcat fits very well to the Gross-Butler equation, with x = 0.825 +/- 0.08, suggesting that only a single proton is transferred in the rate-determining step. In contrast, there is no significant kinetic isotope effect on either kcat or kcat/Km with alpha-[2H]-L-kynurenine as the substrate. There is a "burst" of anthranilate (0.7 mol/mol of enzyme) formed in the pre steady state of the reaction of kynureninase, with a rate constant of 54 s-1 which is not affected by [2H]H2O. The partition ratio of alanine to pyruvate formation is 2.3 x 10(4) in H2O and 6.9 x 10(3) in [2H]H2O. Taken together, these data indicate that the rate-limiting step in the reaction of kynureninase occurs subsequent to the first irreversible step, which is anthranilate release, is general base catalyzed, and involves transfer of only a single proton. On the basis of these observations, we propose that the rate-limiting step in the reaction of kynureninase is C-4' deprotonation of the pyruvate pyridoxamine 5'-phosphate ketimine intermediate.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Georgia, Athens 30602-2556, USA.
authors.forename: S V
authors.initials: SV
authors.lastname: Koushik
authors.forename: J A
authors.initials: JA
authors.lastname: Moore
authors.forename: B
authors.initials: B
authors.lastname: Sundararaju
authors.forename: R S
authors.initials: RS
authors.lastname: Phillips
mesh_terms: D002384:Catalysis; D003903:Deuterium; D017666:Deuterium Oxide / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D006867:Hydrolases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D007700:Kinetics; D007737:Kynurenine / Q000378:metabolism; D011551:Pseudomonas fluorescens / Q000201:enzymology*; D012997:Solvents; D013053:Spectrophotometry; D013379:Substrate Specificity; D062367:ortho-Aminobenzoates / Q000378:metabolism / Q000494:pharmacology
chemical_list: D012997:Solvents; D062367:ortho-Aminobenzoates; C031385:anthranilic acid; D007737:Kynurenine; D003903:Deuterium; D006867:Hydrolases; C021480:kynureninase; D017666:Deuterium Oxide